Supplementary Information for
Solution-Processed PCDTBT Capped Low-Voltage InGaZnOx Thin Film
Phototransistors for Visible-Light Detection
Han Wang†, Yubin Xiao†, Zefeng Chen†, Wangying Xu, Mingzhu Long, Jianbin Xu*
Department of Electronic Engineering and Materials Science and Technology Research
Center, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong, People’s
Republic of China
* jbxu@ee.cuhk.edu.hk
*To whom all correspondence should be addressed.
†There
authors contribute equally to this work.
1. Experimental details
To minimize the device operating voltage, a high-k gate dielectric, Al2Ox thin film
of 45 nm-thickness was fabricated via solution spin coating onto highly doped silicon,
then was annealed at 260oC. IGZO semiconductor thin film was chemical combustion
derived followed the mole ratio of In: Ga: Zn of 65:10:25 with a total concentration of
0.05 mol/L, then was annealed at 260 oC as well. Spin coating and annealing for 4 times.
The detailed fabrication process was documented elsewhere.1 150 nm-thick Al
electrodes were thermal deposited with shadow mask as source and drain to satisfy the
top contact configuration. The field effect mobility (μsat) of the TFTs is extracted in the
saturation region of the transfer curve by the following equation:
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I D,sat 
w
sat Cox (VG  VT ) 2
2L
(S-1)
Where ID is the drain to source current, VG is the gate to source voltage, VT is the
threshold voltage, W and L are the width and length of the channel, respectively, and
Cox is the capacitance per unit area of the gate dielectric. The measured capacitance
density of Al2Ox is 300 nF/cm2 at the frequency of 100 Hz, which is used to calculate
the mobility. The channel length (L) was 100 μm and its width (W) was 1500 μm.
PCDTBT was dissolved in o-1,2-dichlorobenzene (o-DCB) to form a solution with a
concentration of 7 mg/mL, which was then spin coated to cap the top surface of the
traditional transistor, to form a phototransistor. Detailed PCDTBT thin film preparation
process can be found elsewhere.2 The thickness of PCDTBT was 80 nm. To guarantee
the Ohmic contact for electrical measurement, the capped PCDTBT onto the source and
drain electrodes were scratched off by conducting probe under microscope of the probe
station.
To compare the light transmittance performance of IGZO transistor and PCDTBT
capped phototransistor, the films were spin coated on the cleaned glass substrates. The
substrates were solution deposited with Al2Ox dielectric layer firstly and then IGZO
semiconductor layer. One was kept pristine while another was covered with PCDTBT.
For PCDTBT characterization, the PCDTBT solution was spin coated directly on the
glass substrates.
2. Results and Discussion
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Figure S1. The leakage performance of solution processed Al2Ox.
Figure S2. Transfer comparison between pristine IGZO TFT and PCDTBT capped
IGZO TFT
There is an obvious doping effect coming from PCDTBT to the channel of IGZO.
Because of the formation of a depletion region, free carriers closing to the interface of
IGZO and PCDTBT are recombined, resulting in a decreased carrier concentration,
which is evidenced by a positive threshold voltage shift and a reduced drain current.
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Figure S3. The device photoresponse recovery behavior. (a) Behavior to 532 nm-laser.
(b) Behavior to 635 nm-laser.
To specify the device recovery behavior, a large laser power density of 110
mW/cm2 was introduced for characterization. Under a large power density, an enhanced
transfer hysteresis are observed, indicating the generation of a great amount of traps.
The transfer performance was examined immediately just after turning off the laser, the
device recovers almost to its original dark state.
Figure S4. The photoresponse to the halogen lamp
A halogen lamp is also utilized as a more common light source, which is shining
on the device through the microscope objective lens connected to our probe station. The
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photoresponse to the halogen lamp is also detected. We believe the PCDTBT capped
IGZO phototransistor is a good candidate for visible-light detection.
Figure S5. Semi-log graph of (a) photoresponsivity and (b) optical gain of 532 nm-laser
and 635 nm-laser as a function of laser power density, at gate voltage of -0.5 V.
In the depletion regime or near off-state, although the photocurrent is extremely
increased by increasing the light intensity, because the photoresponsivity is mainly
confined by the magnitude of drain current rather than the current difference, the
responsivity and the optical gain is much lower than those performed in accumulation
region. In accumulation region, the current is in the order of microampere, although the
current difference is small, the photocurrent is still very high. In depletion region, the
current is more than one order smaller than that of accumulation region, hence resulting
in a weak responsivity. The responsivity and optical gain at Vg= -0.5 V were calculated.
The 532 nm-laser induces a higher photoresponsivity than the 635 nm-laser as well,
achieving to the maximum of 0.57 A/W at the power density of 0.05 W/cm2, while it is
0.009 A/W for the 635 nm-laser. Comparing to those at Vg= 1 V, the responsivity
become 1/35 and 1/366 for 532 nm-laser and 635 nm-laser, respectively. At Vg= -0.5
V, the calculated optical gain at the power density of 0.05 W/cm2 are 2.35 and 0.04 for
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532 nm-laser and 635 nm-laser, respectively.
Figure S6. Photoresponse under illumination of halogen lamp. (a) Transfer and (b)
Output performances of PCDTBT capped IGZO phototransistor with thermal grown
SiO2 as gate dielectric.
To further demonstrate the PCDTBT capped IGZO phototransistor’s performance
under large negative gate voltage, we utilize thermal grown SiO2 (thickness is 300 nm)
as gate dielectric instead of Al2Ox. Similar light detection behavior is observed,
comparing to the device with solution processed Al2Ox as gate dielectric. In the gate
voltage range of -40 V to -30 V, because of the fully depleted channel and the low
leakage, the drain current is very low and keeps almost unchanged, indicating the
unipolar behavior.
1
Han Wang, Tieyu Sun, Wangying Xu, Fangyan Xie, Lei Ye, Yubin Xiao, Yu Wang, Jian Chen, and
Jianbin Xu,
2
RSC Advances 4 (97), 54729 (2014).
Yubin Xiao, Shuang Zhou, Yaorong Su, Lei Ye, Sai-Wing Tsang, Fangyan Xie, and Jianbin Xu,
Organic Electronics 15 (12), 3702 (2014).
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